1. Field of the Invention
This invention relates to conductive polymer compositions, their preparation, and devices comprising them.
2. Summary of the Prior Art
It is known that polymers, including crystalline polymers, can be made electrically conductive by dispersing therein suitable amounts of finely divided conductive fillers. Some conductive polymers exhibit what is known as PTC (positive temperature coefficient) behavior. The term "PTC" has been used in various different ways in the past, but in this specification, the terms "composition exhibiting PTC behavior" and "PTC" composition" are used to denote a composition which has an R.sub.14 value of at least 2.5 and an R.sub.100 value of at least 10, and preferably has an R.sub.30 value of at least 6, where R.sub.14 is the ratio of the resistivities at the end and the beginning of a 14.degree. C. range, R.sub.100 is the ratio of the resistivities at the end and the beginning of a 100.degree. C. range, and R.sub.30 is the ratio of the resistivities at the end and the beginning of a 30.degree. C. range. A plot of the log of the resistance of a PTC element (i.e. an element composed of a PTC composition) against temperature will often show a sharp change in slope over a part of the temperature range in which the composition has an R.sub.100 value of at least 10. The term "switching temperature" (usually abreviated to T.sub.s) is used herein to denote the temperature at the intersection point of extensions of the substantially straight portions of such a plot which lie either side of the portion showing the sharp change in slope. The term "peak resistivity " is used herein to denote the maximum resistivity which the composition exhibits above T.sub.s, and the term "peak temperature" is used to denote the temperature at which the composition has its peak resistivity.
Recent research relating to conductive polymers is described in, for example, U.S. Pat. No. 3,858,144, copending and commonly assigned Applications Ser. Nos. 601,638 (Horsma et al), 750,149 (Kamath et al), 751,095 (Toy et al), 798,154 (Horsma), 873,676 (Horsma) and 943,659 (van Konynenburg), and the application Ser. No. 965,344 of Middleman et al. entitled Circuit Protection Devices comprising PTC elements and the application Ser. No. 965,345 of Middleman et al entitled PTC Devices Comprising Oxygen Barrier Layers, both filed contemporaneously with this application. The disclosures of this patent and these applications are incorporated by reference herein.
Particularly useful known PTC compositions comprise a thermoplastic crystalline polymer with carbon black dispersed therein. The polymers which have been used include polyolefins, e.g. polyethylene, and copolymers of olefins and polar comonomers. Generally the composition is cross-linked, preferably by irradiation at room temperature, to improve its stability at temperatures above T.sub.s. At present, the only commercial use of such compositions is in self-regulating heaters, and for this use, the compositions must have a relatively high resistivity at room temperature, usually at least 10.sup.3 ohm. cm. It has been recognised that there are important potential uses for PTC conductive polymer compositions having much lower resistivities at room temperatures, e.g. below 10 ohm. cm, preferably below 7 ohm. cm, especially such compositions which are electrically stable when exposed to elevated temperature. [See for example U.S. Pat. Nos. 2,978,665 (Vernet et al) and U.S. Pat. No. 3,243,753 (Kohler)]. However, the preparation of such compositions has presented very serious problems. For example, it has been found that as the content of conductive filler in a PTC conductive polymer composition has been increased, in order to reduce the resistivity of the composition, there has been a sharp reduction in the intensity of the PTC effect [see for example M. Narkis et al, Poly Eng and Sci, 18, 649 (1978)]. In addition, it has been found that when PTC conductive polymer compositions are exposed to elevated temperatures, their resistivity increases sharply [see for example J. Meyer, Poly Eng and Sci., 14, 706 (1974)].